The present invention relates to a photoelectric scanning apparatus for a facsimile system or the like comprising a charge accumulation type photosensor array such as a charge coupled device (CCD) array or a photo diode array (PDA).
In a device of the present type, which is disclosed in detail in copending U.S. patent application Ser. No. 967,834, filed Dec. 8, 1978 entitled "FACSIMILE TRANSCEIVER UTILIZING ORTHOGONAL CONVERSION" which is assigned to the same assignee as this application, a CCD photosensor array or a PDA is reciprocated relative to an original document for scanning the same. The direction of movement is perpendicular to the row of photosensor elements which make up the array. A timing pulse generator produces timing signals in response to each incremental movement of the array. Each time a timing signal is produced, all of the photosensor elements produce data signals at their outputs in a serial manner. The magnitude of each data signal corresponds to the intensity of incident light from the document.
Since a CCD array produces the data signals which depend not only incident light but also on the charge accumulation time (the time intensity is integrated with respect to time), the output of the array depends on the period of the timing pulses. At the beginning of each stroke of the reciprocating movement the array is accelerated and the periods of the timing pulses progressively decrease. The opposite occurs at the end of each stroke. For this reason, the array produces data signals whose magnitudes are maximum at the ends of the strokes and minimum in the middles.
For this reason, a servo motor or the like which drives the array is designed to accelerate to a predetermined speed in a minimum distance and thereafter move the array at constant speed. At the end of the stroke, the motor decelerates the array in a minimum distance to a stop. The object of this design is to provide a constant speed area which occupies the maximum possible range of the movement of the array. In the constant speed area, the periods of the timing pulses are constant and therefore the data signals produced by the array vary only proportion to the intensity of incident light.
It has been proposed in the prior art to generate a data enable signal while the array is moving in the constant speed area and furthermore give the constant speed area a predetermined length such as corresponding to 1728 timing pulses. Where the array has 64 elements, a total of 1728.times.64=110,592 data signals will be produced during each scan stroke of the array. A gate is provided so that the timing pulses will be applied to the array only while the enable signal is produced and thereby the array can only produce the data signals in the effective area. However, a problem has been heretofore unsolved in that the first 64 data signals have maximum magnitude corresponding to a saturated condition of the photosensor elements. This is because charge accumulation has occured in the array from the beginning of the stroke to the beginning of the effective area. This results in a perfectly white line constituting the first incremental area of the scan.
A prior art proposal to overcome this problem involves extending the effective area to 1729 increments and ignoring the first 64 data signals. However, this requires complicated counter and gating circuitry which adds unnecessarily to the cost of the apparatus.